The development of morphologic complexity has been measured using tetanus toxin immunohistochemistry and analysis of fractal dimension in dissociated cultures of mouse spinal cord. Increases in complexity with time are non-linear, the most rapid development occurring over the initial 24 hr after plating. Functional rFragment C of tetanus toxin, cloned and expressed after using the polymerase chain reaction, appears to be a suitable alternative to tetanus toxin or toxin-derived fragment C for studies of its role in toxin binding and internalization, and for use in immunohistochemistry.